1. Field of the Invention
The present invention relates to chloride-free set accelerated cement compositions and methods of using the compositions in oil and gas well cementing operations.
2. Description of the Prior Art
In the primary cementing of oil and gas wells, a hydraulic cement composition is pumped down the well bore penetrating a subterranean formation through a conduit disposed in the well bore and back up through the annulus between the conduit and the well bore. Upon setting, the cement composition bonds to the conduit and to the sides of the well bore whereby the conduit is supported within the well bore and fluid movement between subterranean formations penetrated by the well bore is prevented. To be useful as a well cementing composition, the composition, among other things, must retain its pumpability for a sufficient period of time, have a low viscosity while being pumped, resist fluid loss into subterranean formations and, after placement, rapidly thicken, set and develop strength.
In a number of primary cementing applications it is necessary to include a set accelerator in the cement compositions as a result of the set time retardation caused by low temperatures and the use of fluid loss reducing additives, dispersants, salts and the like. For example, high alkali metal or alkaline earth metal salt concentrations are often utilized in cement composition used in primary cementing applications. The salt functions as a dispersant in the cement composition, and can cause the composition to expand upon setting whereby the attainment of a good bond between the walls of the well bore and the conduit being cemented therein is enhanced. In addition, the use of a high salt concentration cement composition contributes to preventing the swelling of clays which are contacted by the composition and the sloughing of salt-containing shales which in turn prevents problems such as bridging and lost circulation. In applications where high salt concentration cement compositions are used, i.e., concentrations of from about 18% by weight of water to saturation, and the temperatures to which the compositions are exposed when setting are relatively low, the cement composition set times can be excessive. The use of conventional fluid loss reducing additives in combination with high salt concentrations can extend the set times of cement compositions even longer.
While a variety cf set time accelerator additives have been developed and used heretofore, a number of such additives have been comprised of one or more chlorides. Such chloride-containing set accelerators in cement compositions can cause corrosion of the metal casing and liners in contact therewith, can reduce the sulfate stability of the cement and bring about other adverse results.
In one application of the present invention, chloride-free set time accelerators, chloride-free cement compositions containing such accelerators and methods of using the compositions in oil and gas well cementing operations are provided. In another application of the present invention, chloride-free set time accelerators are employed to negate the retarding effects of high concentrations of salt or other cementing additives.